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I can't seem to get into the area to rescue Zoya's sister because I don't have his letter. Is there a way to regen it?
You mean the pass to his den? It's not sellable. No. You'd need to use an earlier save file. Check the places where you normally stash things.
Hmm... Maybe I'm wrong on this... If the letter comes from rescuing his sister, then my problem is that I can't find her.
The letter comes from Zoya. It's called Pass to Solovey's Den. You get it from him when you find her. So that's your problem, imho. Alas, however, she was eaten by a wolf who drops her necklace--Shura's Necklace. That's what Zoya is looking for. Wolf who ate her is in the Wolves' Den.